The present invention relates to a process and a device for the production of shaped foamed polyurethane articles by introducing an expandable polyurethane reactive mixture into a mould divided by a parting plane, wherein the parting plane has a circumferential vacuum channel through which the mould cavity defined by the mould halves is evacuated. There have already been a number of proposals for this; see e.g. DE-A 15 04 278, DE-A 30 20 793 and EP-A 23 749.
Evacuation of the mould is necessary in particular in order to remove the gas present in the mould from the mould to avoid the formation of voids. To achieve this, it is necessary to construct the mould halves such that the parting plane lies at the highest point of the mould cavity, since otherwise a pocket forms from which the gas present can no longer can be drawn off. This could be counteracted by evacuating the mould cavity to a very low pressure of below 100 mbar, in particular below 50 mbar, before the foam rises, i.e. before the level of the foam in the mould cavity passes beyond the parting plane. However, such a low pressure in the mould cavity means that the foam initially expands so rapidly and severely, before substantial contents of blowing agent have yet been released, that only an irregular foam structure forms.
Another disadvantage of evacuation via the parting plane is that the delivery via the parting plane is relatively low, especially if the expandable reactive mixture is introduced into the open mould and evacuation is carried out only after the mould has been closed, so that very long cycle times are necessary.
The properties of polyurethane foam are substantially determined by the bulk density of the finished foam and the material properties of the matrix. If water is employed as a chemical blowing agent in particular, in which case carbon dioxide is liberated by the reaction of the water with the isocyanate, the recipe of the expandable polyurethane reactive mixture must be adjusted precisely to establish the matrix properties. It is therefore in principle desirable to be able to produce foams of different bulk densities with a single recipe. Controlling the pressure in the foam mould is outstandingly suitable for controlling the bulk density of a foam while simultaneously retaining the recipe, especially of the blowing agent content, see e.g. EP-A 23 749 and EP-A 44 226.
The object of the present invention is to produce, over short cycle or sequence times, void-free mouldings of lower bulk density than is to be expected on the basis of the blowing agent content under atmospheric pressure, it also being possible to employ contoured mould cavities in which the parting plane cannot be placed at the points of maximum height of the mould cavity for geometric reasons.